The present invention generally relates to fluid sensing devices and methods of using such devices. More particularly, this invention relates to a fluid system installed on a vehicle and the use of a micromachined tube to assess one or more properties of a fluid flowing in the fluid system.
A variety of sensors have been used in land-based, aerospace, and aquatic-based vehicles to enhance engine performance and safety. For example, pressure sensors are used in a variety of passenger vehicles to monitor engine air intake, engine cylinder, engine oil, and tire pressures, and to detect fuel vapor leaks. Other examples include the use of hot-wire flow sensors and drag-force sensors to measure air intake flow rates, oxygen sensors to monitor oxygen levels in engine exhaust gases, and accelerometers and gyroscopes used in vehicle safety systems. More recently, certain alternative fuels used in automotive applications require the use of fuel systems capable of monitoring fuel constituents. Notable examples include flex fuels that contain about 85% ethanol (E85) or methanol (M85) with the balance (about 15%) essentially gasoline, and reformed fuel cells that often employ alcohol/water solutions. Measuring the concentration of two liquid fuel constituents is a common theme in these alternative fuel systems. For example, flex fuel engines benefit from the ability to distinguish between pure gasoline and the alternative fuel (E85 or M85). If gasoline and the alternative fuel are mixed in the same fuel tank, a measure of the ethanol/methanol or gasoline concentration can be used to optimize the engine operating parameters for performance. A capacitive sensor is disclosed in U.S. Pat. No. 5,497,753 as being capable of such use. However, sensor with improved concentration resolution would be desirable.